The present invention relates to aqueous liquid detergent compositions which contain sufficient detergent active material and, optionally, sufficiently dissolved electrolyte to result in a structure of lamellar droplets dispersed in a continuous aqueous phase.
Lamellar droplets are a particular class of surfactant structures which, inter alia, are already known from a variety of references, e.g., H. A. Barnes, `Detergents`, Ch. 2 in K. Walters (Ed), `Rheometry: Industrial Applications`, J. Wiley & Sons, Letchworth 1980.
Such lamellar dispersions are used to endow properties such as consumer-preferred flow behaviors and/or turbid appearance. Many are also capable of suspending particulate solids such as detergency builders or abrasive particles. Examples of such structured liquids without suspended solids are given in U.S. Pat. No. 4,244,840 to Straw, whilst examples where solid particles are suspended are disclosed in specifications EP-A-160,342; EP-A-38,101; EP-A-104,452 and also in the aforementioned U.S. Pat. No. 4,244,840. Others are disclosed in European Patent Specification EP-A-151,884, where the lamellar droplet are called `spherulites`.
The presence of lamellar droplets in a liquid detergent product may be detected by means known to those skilled in the art, for example optical techniques, various rheometrical measurements, X-ray or neutron diffraction, and electron microscopy.
The droplets consist of an onion-like configuration of concentric bi-layers of surfactant molecules, between which is trapped water or electrolyte solution (aqueous phase). Systems in which such droplets are close packed provide a very desirable combination of physical stability and solid suspending properties with useful flow properties.
The viscosity and stability of the product depend on the volume fraction of the liquid which is occupied by the droplets. Generally speaking, the higher the volume fraction of the dispersed lamellar phase (droplets), the better the stability. However, higher volume fractions also lead to increased viscosity which in the limit can result in an unpourable product. This results in a compromise being reached. When the volume fraction is around 0.6, or higher, the droplets are just touching (space-filling). This allows reasonable stability with an acceptable viscosity (say no more than 2.5 Pas, preferably no more than 1 Pas at a shear rate of 21s.sup.-1). This volume fraction also endows useful solid suspending properties. Conductivity measurements are known to provide a useful way of measuring the volume fraction, when compared with the conductivity of the continuous phase.
FIG. 1 shows a plot of viscosity against lamellar phase volume fraction for a typical composition of known kind:
______________________________________ wt. % ______________________________________ Surfactants* 20 Na-formate 5 or 7.5 Na-citrate 2aq 10 Borax 3.5 Tinopal CBS-X 0.1 Perfume 0.15 Water Balance ______________________________________ *For example, a system comprising Nadodecyl benzene sulfonate, lauryl ether sulfate and C.sub.12 -C.sub.13 ethoxylated, 6.5 EO.
It will be seen that there is a window bounded by lower volume fraction of about 0.65 corresponding to the onset of instability and an upper volume fraction of 0.83 or 0.9 corresponding to a viscosity of 1 Pas or 2 Pas, respectively. This is only one such plot and in many cases the lower volume fraction can be 0.6 or slightly lower.
A complicating factor in the relationship between stability and viscosity on the one hand and, on the other, the volume fraction of the lamellar droplets is the degree of flocculation of the droplets. When flocculation occurs between the lamellar droplets at a given volume fraction, the viscosity of the corresponding product will increase owing to the formation of a network throughout the liquid. Flocculation may also lead to instability because deformation of the lamellar droplets, owing to flocculation, will make their packing more efficient. Consequently, more lamellar droplets will be required for stabilization by the space-filling mechanism, which will again lead to a further increase of the viscosity.
The volume fraction of droplets is increased by increasing the surfactant concentration and flocculation between the lamellar droplets occurs when a certain threshold value of the electrolyte concentration is crossed at a given level of surfactant (and fixed ratio between any different surfactant components). Thus, in practice, the effects referred to above mean that there is a limit to the amounts of surfactant and electrolyte which can be incorporated whilst still having an acceptable product. In principle, higher surfactant levels are required for increased detergency (cleaning performance). Increased electrolyte levels can also be used for better detergency, or are sometimes sought for secondary benefits such as building.
The dependency of stability and/or viscosity upon volume fraction can be favorably influenced by incorporating a deflocculating polymer comprising a hydrophilic backbone and one or more hydrophobic side chains. The use of such a polymer is taught in U.S. Pat. No. 5,147,576 to Montague et al., hereby incorporated by reference.
Another approach to viscosity control is to formulate the structured liquids to be shear thinning, i.e., accepting the high viscosity of the product at rest in a bottle but devising the composition such that the action of pouring causes shear beyond the yield point, so that the product then flows more easily. This property is utilized in the compositions described in our aforementioned specification EP-A-38,101. Unfortunately, it has been found that this cannot easily be utilized in liquids with high levels of active.
Yet another approach to controlling viscosity in aqueous structured liquids is use of viscosity reducing polymers such as taught in U.S. Pat. No. 5,108,644 to Machin et al. hereby incorporated by reference.
In none of these patents is it taught or suggested that viscosity control (as well as other benefits such as draining characteristics) can be controlled through process variables.